This invention relates generally to the control of single sideband communications systems and more particularly to coded squelch transmission and coded squelch receiver enablement in a single sideband system.
Single sideband (SSB) modulation of radio frequency carriers has been known and used for many years. Its principal advantage is a reduction in occupied bandwidth over amplitude modulation or angle modulation. SSB is, in reality, amplitude modulation with a carrier and one of the two sidebands suppressed, utilizing only one sideband to convey information from the transmitter to the receiver. The receiver, to demodulate the SSB signal, must recreate the suppressed carrier with the same frequency and phase relationship to the single sideband as that of the original carrier in order to prevent distortion of the demodulated information. Voice information is tolerant of some frequency error in the recreated carrier and empirical studies have indicated that "natural" speech is obtained in receivers having a frequency error of less than 20 Hz. Frequency errors greater than this result in the characteristic "Donald Duck" or "Monkey chatter" speech frequency distortion of an untuned single sideband receiver.
To further complicate SSB reception, the strength of the radio frequency signal received by a receiver varies with propagation conditions which can change rapidly at VHF and UHF frequencies. These variations in radio frequency signal level result in corresponding variations in the detected audio level, causing a degradation of intelligibility in the recovered signal. Consequently, automatic gain control (AGC) circuits are routinely provided in single sideband receivers to maintan a constant recovered signal level.
Commercial and amateur radio operators are familiar with the fine tuning required to maintain an acceptable frequency error. Citizen's band radios introduced a greater segment of the public to the "clarifier" control of single sideband citizen's band radios which allows the manual adjustment of the receiver frequency of a crystal controlled radio. To make frequency control easier for other operators who do not want the annoyance of a manual control, an automatic frequency control (AFC) has been developed employing a "pilot" signal to establish a frequency reference.
The pilot signal is transmitted continuously with the single sideband message and is used by the receiver to vary the frequency of an internal mixing signal to track and eliminate any frequency variation imposed on the single sideband signal by either the transmission medium or the system. The pilot may also be used as a reference signal for the Automatic Gain Control. The gain of the receiver amplifying circuitry is adjusted so as to maintain a constant detected pilot level which thereby ensures that the audio output from the receiver does not vary.
The pilot signal may be a "zero-frequency" carrier or an audio tone within the modulation bandwidth. The generation of a carrier pilot is simply done by providing a controlled leakage path around the transmitter sideband filter. This leakage is typically adjusted to a level 5-15 decibels below the peak sideband power. The audio tone pilot requires a stable oscillator, the output of which is also typically set so as to produce a signal level which is set to be 5-15 decibels below the peak sideband power. The pilot signal should not interfere with the single sideband information nor should it constitute a significant part of the transmitter output power.
The SSB signal may experience frequency variations which are caused by equipment frequency offsets due to temperature drift, aging, and setting errors; shift due to relative motion of the transmitter and receiver (common in transmission between two aircraft); random frequency modulation due to multipath propagation between the transmitter and receiver; and purposeful variations intentionally introduced in the system. It is well known that these diverse forms of frequency variation may be "tracked" by a phase locked loop or other type of frequency tracking system, as long as the bandwidth of the frequency variation lies substantially within the bandwidth of the tracking circuit.
One feature of land mobile radio systems, which is valuable in directing communications to a selected one or group of users, is that of tone or digitally coded squelch. However, since the frequency spectrum of the coded squelch signal generally falls below the audio passband which begins at approximately 300 Hz, practical SSB filters employed in the suppression of the carrier and undesired sideband distort and suppress the coded squelch signal. Therefore, the transmission of the coded squelch signal at frequencies below the modulated passband is impractical in SSB transmitters. To resolve this difficulty, others have angle modulated and audio frequency pilot tone with the coded squelch signal. This, however, introduces a new set of problems.
A receiver typically utilized the pilot tone for AFC by phase-comparing the incoming pilot with an internal tone frequency reference. The control signal generated by the phase comparator is then used to vary the frequency of a local oscillator stage which drives a frequency convertor in the receiver. Since the pilot tone is also used for the coded squelch signal, the phase locked loop frequency response must be made lower (longer time constant) than the frequency of the coded squelch signal so that the AFC loop does not track the modulated coded squelch signal. Typical frequency response corners range over the decade of 2 Hz to 20 Hz. If this tracking were to occur, the coded squelch signal would be introduced into the demodulated single sideband message where audio mixing can produce undesirable in-band interference. A long time constant AFC loop, however, cannot track the higher frequency forms of undesirable frequency variations introduced by multipath signal propagation and therefore cannot eliminate them from the single sideband signal. Moreover, such an AFC system precludes the detection of digital NRZ FSK modulation, since the frequency response of this frequently-used form of modulation has its peak value at OHz. Consequently, the AFC time constant can never really be sufficiently low to avoid introducing undesirable in-band interference.
The present invention provides a means by which coded squelch signals can be transmitted together with a SSB voice signal in systems using either the carrier or an audio tone as a pilot signal. Others have previously used an angle modulated carrier in single side-band systems, but only for frequency control and purposes other than carrying information to be demodulated, decoded, and used by the receiver. The conditions under which coded squelch may be transmitted by a SSB system are disclosed hereinbelow.